Light in a mid-infrared wavelength region (for example, a wavelength of 5 to 30 μm) is a wavelength region important in the field of spectroscopic analysis. As a high-performance and compact semiconductor light source in this wavelength region, in recent years, quantum cascade lasers (QCL) have gained attention (for example, refer to Patent Documents 1 to 6 and Non-Patent Documents 1 to 3).
A quantum cascade laser is a monopolar type laser element which uses a level structure including subbands formed in a semiconductor quantum well structure and generates light by means of intersubband electron transition, and can realize high efficiency and high-output operations by multistage cascade-coupling of quantum well emission layers which are formed by quantum well structures and become active regions. Further, this cascade coupling of quantum well emission layers is realized by using electron injection layers for injecting electrons into emission upper levels and alternately laminating quantum well emission layers and injection layers,    Patent Document 1: Japanese Patent Application Laid-Open No. H8-279647    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-032691    Patent Document 3: Japanese Patent Application Laid-Open No. 2004-119814    Patent Document 4: U.S. Pat. No. 5,457,709    Patent Document 5: U.S. Pat. No. 5,509,025    Patent Document 6: Japanese Patent Application Laid-Open No. H10-4242    Non-Patent Document 1: M. Beck et al., “Continuous Wave Operation of a Mid-Infrared Semiconductor Laser at Room Temperature,” Science Vol. 295 (2002) pp. 301-305    Non-Patent Document 2: J. S. Yu et al., “High-power continuous-wave operation of a 6 μm quantum-cascade laser at room temperature,” Appl. Phys. Lett. Vol. 83 (2003) pp. 2503-2505    Non-Patent Document 3: A. Evans et al., “Continuous-wave operation of λ˜4.8 μm quantum-cascade lasers at room temperature,” Appl. Phys. Lett. Vol. 85 (2004) pp. 2166-2168